The article of Hagenauer, J. and Stockhammer, T. “Channel Coding and Transmission Aspects for Wireless Multimedia”, Proceedings of IEEE, Vol. 87, No. 10, October 1999, discloses joint source/channel coding and decoding methods for multimedia. Multimedia has to handle a variety of compressed and uncompressed source signals such as data, text, image, audio and video. On wireless channels the error rates are high and joint source/channel coding and decoding methods are advantageous.
In the heterogeneous world of communication the layered structure is an important feature for standardization, design and implementation. Usually one layer only communicates with the corresponding layer at the receiver side by using the lower layers to fulfil the requests of the upper layer. For both standardization and implementation, only the definition of interfaces and tasks for each layer is necessary, whereby the interface definition is quite simple. The layer is usually described using a state machine. There also exists a very clear separation in the layer model: end-to-end applications are transported over different physical media like optical fibre, copper wires or wireless within one connection.
In contrast to the layer structure, an optimisation of compression and transmission stretching across the layers might be useful in the mobile environment. The source coding scheme and even the application control could be influenced by the state of the mobile channel and the available resources. Some services might be restricted because of error, complexity and delay constraints. Communication systems optimised to both the application and channel might be of interest in the future for very bandwidth and power efficient transmission.
If there is some knowledge about the source properties, i.e. bit sensitivity measurements or source significant information, or if the application provides base information separated from enhancement information, Unequal Error Protection (UEP) methods should be applied by using advanced channel coding algorithms or modulation techniques. The more important base information is highly protected to guarantee delivery and the less important enhancement information is either low protected or in bad channel connections even not transmitted.
One way of indicating such different types of protection is by using SSI (Source Significant Information) fields in the transmitted data. PCT-application EP01/07759 filed May 07, 2001 describes the insertion of such fields in transmitted data. Here an SSI header is placed before each packet of source data. The SSI header contains the sizes of the partitions having different needs for protection and the code rate to be used for protecting the partitions. These SSI headers are in some cases preceded by a pseudo-random word in the form of a pseudo-noise sequence: Reference is here made to John G. Proakis, “Digital communications”, 2nd edition, McGraw-Hill, 1989, pp. 601–817, pp. 831–836. A preferred pseudo-noise sequence is a Gold sequence, which is known in the art. Pseudo-noise sequences have auto-correlation properties particularly suitable for detection and/or synchronization.
The problem with these SSI headers is that they are inserted into data packets at one high level layer in the communication structure, while the coding using UEP protection is done at a lower level. Because of the limited size of the data packets at the lower level, the high-level data packets can be split into different low-level packets. This phenomenon is also called packet fragmentation. If this happens the receiver can then be unable to decode a data packet coded with UEP, since the information regarding how the data in the packet is to be decoded would be included in another packet. This can thus lead to a loss of information on the receiver side. There is thus a need for a more robust coding scheme, where the negative results of packet fragmentation is limited or completely avoided.